1. Field of Invention
The invention relates to an oscillator circuit for a sensor, with a tuned circuit and an operational amplifier, the electrical oscillation of the tuned circuit capable of being tapped between a first terminal and a second terminal of the tuned circuit and the first terminal of the tuned circuit being connected to the noninverting input of the operational amplifier and the output of the operational amplifier being fed back to the noninverting input of the operational amplifier.
2. Description of Related Art
Oscillator circuits of the type under consideration here have long been known in different embodiments and are used in circuit engineering wherever electrical oscillations (generally periodic characteristics of electrical voltages) are used, and for example, they are used as clocks for a circuit as a carrier of information or as periodic test signals.
One common application for these oscillator circuits is use in proximity sensors, for example, in measurement devices and proximity switches with an inductive, capacitive and/or resistive sensor element. The necessity of using oscillators here is based on the effect that the electrical oscillation caused by a tuned circuit likewise changes when certain characteristics of the tuned circuit which can be influenced from the outside change, and this change of oscillation can be used for further evaluation.
The conventional use of an oscillator circuit is explained below using the example of an inductive proximity sensor; however, analogous effects can also be observed in capacitive or resistive sensors or sensor elements.
Oscillator circuits conventionally have a tuned circuit, such as, for example, a harmonic tuned circuit in the form of a LRC network. The electrical oscillation of the tuned circuit is conventionally amplified by means of an amplifier circuit and looped back again to the tuned circuit, so that the tuned circuit, under certain conditions, tends to sustained oscillation. For proximity sensors, either the capacitor or the coil of the tuned circuit is made such that the capacitance of the capacitor or the quality of the coil can be easily influenced by a conductive article, for example, a metal part, approaching the respective sensor. In the case of an inductively operating sensor, the approach of a conductive article to the sensor causes the stray field of the coil of the tuned circuit to induce an eddy current in the conductive article which takes energy from the electromagnetic field of the coil, and thus, attenuates the tuned circuit. The resulting change in amplitude of the electrical oscillation of the tuned circuit is thus an indicator for the change of the quality of the tuned circuit and it is indirectly a measure of the approach of the conductive article to the sensor itself. The attenuation of the tuned circuit in the simple oscillator circuit under discussion here leads to the electrical oscillation of the tuned circuit coming completely to a standstill. The structure and manner of operation of the inductive sensor is known and is common in industrial use (Schiff, A.: Inductive and Capacitive Sensors, The Library of Engineering, vol. 24, Verlag Moderne Industrie, 1989; Tietze, U., Schenk Ch.: Semiconductor Circuit Engineering, 12th edition. Springer Verlag, 2002, page 874).
As is recognized, a sustained oscillation only occurs when the feedback signal has the amplitude of the input signal, i.e., when the amplifier compensates for possible transmission losses and when the positive feedback signal in the case of positive feedback is in phase with the input signal, or in the case of negative feedback is phase-shifted 180° to this signal. The described eddy current losses reduce the total gain of the oscillator circuit, by which, ultimately for the case in which the conductive article is in the influence area of the electromagnetic stray field, the oscillation condition for the tuned circuit is no longer satisfied, and therefore, the electrical oscillation comes to a standstill.
The oscillator circuit which is described here and which is also known in modifications inherently has certain disadvantages, but especially in conjunction with the described sensors. One disadvantage is the inevitable chopping of the electrical oscillation of the oscillator circuit. Since oscillators require a certain build-up time in order to shift from a nonoscillating state into an oscillating state, such a sensor or the electrical oscillation of the oscillator circuit used in it is not suited for communicating its actual influence state at any time. Therefore, the sensors cannot be used correctly for a certain time; this especially means a considerable limitation for the serviceability of such an inductive sensor when, for example, high-speed movements of articles, for example, in a counting process, must be recognized.